Vector signal analyzers are useful in measuring and demodulating digitally modulated signals like W-CDMA, LTE, and WLAN. These measurements are used to determine the quality of modulation and can be used for design validation and compliance testing of electronic devices. A vector signal analyzer is an instrument that measures the magnitude and phase of the input signal centered at a single frequency within the IF bandwidth of the instrument. The primary use is to make in-channel measurements, such as error vector magnitude, code domain power, and spectral flatness, on known signals.
An important design parameter of modem wireless systems is a quantity known as the Error Vector Magnitude (EVM). When a wireless transmitter is being evaluated, EVM data is typically gathered near the final stage in an RF transmitter. EVM is a measure of the distortion detected at the transmitter. This distortion can result in errors in the interpretation of the modulated symbols. These errors alter the transmitted bit stream when the receiver tries to reconstruct the original signal. EVM can therefore be used as a metric for identifying when distortion at the transmitter can cause bit error degradation at the output of the demodulator.
A received symbol may not correspond precisely to the ideal symbol shown in the constellation diagram due to noise associated with the communication channel and imperfections of both the transmitter and receiver. The difference between a received symbol and an ideal symbol can be represented as an error vector. Generally, the smaller the magnitude of the error vector, the better the performance of the communication system. Error vector magnitude is the root mean square (RMS) magnitude of the error vector over time at the precise time instance of the symbol clock transitions. EVM is typically normalized to either the amplitude of the outermost symbol, or the square root of the average symbol power. Each symbol may be represented as a particular amplitude and phase. Thus, the transmitted signal may vary in amplitude and/or phase to transmit a string of consecutive symbols. The amplitude and phase components of a signal may be processed separately in a transmitter.
Compensation via equalization of linear distortions found in transmit and receive RF signal paths can be necessary to meet distortion specifications and provide performance margin for high density signal constellations. Full compensation requires adjustments to both the amplitude and phase of a signal. Determination of the necessary adjustment of amplitude and phase normally requires separate transmit and receive systems or a Full Duplex system capable of transmitting a waveform with amplitude and phase variations as well as sampling both the amplitude and phase.
Current state of the art requires the distorted transmit signal to be sampled via an external Vector Signal Analyzer or via special embedded circuitry that allows for Full Duplex Sampling of I/Q (Inphase and Quadrature) components of the transmitted signal. The hardware to do this is relatively expensive (e.g. Vector Signal Analyzer) or burdensome to design goals of size/weight/power (e.g. full duplex sampling of I/Q).